Thermal switch



Patented Sept. 7, 1943 THERMAL SWITCH Leo R. Peters, Cleveland Heights, hio,` assignor to General Electric Company, a corporation o! New York Application July 1, 1941, Serial No. 400,645

(Cl. G-113) 13 Claims.

My invention relates to thermal switches, and in particular to switches comprising a thermally responsive element actuated by heat generatedr by a gaseous electric discharge. A species of switches of this type is disclosed in my copending application Serial No. 385,282, led March 26, 1941, and another species thereof is disclosed in the United States Patent No. 2,280,550, issued April 21, 1942, on the application Serial No. 289,897, led August 12, 1939, inventor Wayne C. Smitley. This patent and my application are assigned to the assignee of the present application and disclose switches comprising a stationary electrode and a thermally responsive, bimetallic electrode constructed and arranged to extinguish the discharge between said electro-des by closing the gap therebetween when heated by said discharge. On cooling, the bimetallic electrode returns to its original spaced position with respect to the stationary electrode.

These switches are used commercially for controlling the starting of gaseous electric discharge lamps of the positive column type having electrodes which require heating before starting a discharge therebetween. The switch is connected to one terminal of each of the lamp electrodes, and the other terminal of each of said lamp electrodes is connected to a terminal of an alternating current source of commercial frequency. An inductance is connected between a terminal of the source and an electrode terminal. When potential is applied to the circuit, the glow discharge starts in the switch, the gap between the switch electrodes is closed by the flexing of the bimetallic electrode to extinguish said glow discharge and to permit a heavier heater current to ow through the lamp electrodes which are connected in series across the terminals of the current source by the switch. The extinguishing of the glow discharge permits the bimetallic electrode to cool and thus to iiex back to its original, spaced position with respect to the stationary electrode, thus breaking the series connection between the lamp electrodes after these have been heated to their operating temperature and impressing across the lamp the transient voltage surge produced by the inductance. If the discharge in the lamp starts, the voltage then across the switch is the operating voltage of the lamp which is appreciably lower than the voltage of the current source.

Obviously, in such circuits the voltage required to start the discharge in the switch (hereinafter called the breakdown voltage of the switch) must be lower than the voltage of the current source and higher than the operating voltage of the lamp if the switch is to successfully start the lamp Without interrupting the operation thereof after it has started. The breakdown voltage of such switches depends upon the ionizable gas used, the pressure of the gas, the work function of the surface of the electrodes and the spacing of the electrodes.

I have observed that the breakdown voltage is frequently too high or too low in spite of the most careful control of the above factors in the manufacture of the switch. I have discovered that the cause of the difficulty is the mounting of the bimetallic electrode with the free end portion thereof in such position with respect to the stationary electrode that the movement of said bimetal electrode in response to changes in ambient temperature changes the spacing of the electrodes which, in turn, changes the breakdown voltage of the switch. For example, when the free end of the bimetallic electrode is spaced a sufficient distance from the stationary electrode to obtain the desired breakdown voltage at an ambient temperature of approximately 25 C., the spacing is considerably less when the ambient temperature is approximately 100 C. and, of course, the breakdown voltage of the switch is lower. Again, in the manufacture of the Switch, the practice is to fix the spacing of the electrodes before the switch is sealed. The heating of the switch parts incident to the sealing operation ilexes the bimetallic electrode which frequently presses against the stationary electrode with such force that it becomes distorted in shape. On cooling, the spacing of the electrodes is greater than the original spacing thereof and the breakdown voltage of the completed switch is higher than that desired.

An object of my invention is to provide a glowtype thermal switch in which the above diiiculties are avoided. Another object of my invention is to provide a thermal switch of the gaseous electric discharge type in which the breakdown voltage is not affected by changes in ambient temperature encountered in use. A further object of my invention is to provide such a switch of simple structure and minimum cost. Other objects and ladvantages of my invention will appear from the following detailed description thereof and from the appended claims.

I have found that the above objects of my invention are attained when the electrodes are constructed and arranged in such manner that the smallest gap therebetween' is not closed nor appreciably changed in size when the bimetallic electrode flexes in response to changes in ambient temperature encountered in the use of the switch.

This gap is called the fixed gap hereinafter to distinguish it from the. gap between the electrodes which is closed by the flexing of the bimetallic electrode on the occurrence of a discharge between said electrodes. This last gap is called the contact gap hereinafter. My novel structure is utilized to make the breakdown voltage of the switch independent of changes in the ambient temperature over a predetermined range. The ambient temperature range may be selected to that encountered in the manufacture and the use of the device to avoid the dificulties characteristic of prior switches.

In the drawing accompanying and forming part of this specification, species of glow-type thermal switches embodying my invention are illustrated, in which Fig. 1 is a perspective view' of the switch; Fig. 2 is a side elevational, fragmentary view of the stem part thereof; Fig. 3 is a .similar view of the stem part of another species oi switch; and Fig. 4 is a perspective view of a pair of electrodes constructed eind arranged in accordance with the present invention.

The switch illustrated in Figs. 1 and 2 of the drawing comprises a hermetically sealed glass envelope I having a re-entrant stem I2 through which current leading-in wires I3 and I4 are hermetically fused or welded. The stern I2 is provided with an exhaust tube I 5 through which the envelope is exhausted of air and filled with the desired ionizable gaseous atmosphereduring the manufacture of the switch. A U-shaped electrode I8 comprising a thermally responsive, bimetallic element is welded to support I6, such as a nickel rod, connected to wire I 4. The free end of said electrode I8 is located between said support I6 and the electrode I1, said free end terminating near the top of the press of said stem I2 and the lower part of the elongated electrode I1. Said electrode II comprises a metal rod, such as a nickel rod, connected to said wire I3. A refractory metal contact I9, such as a rod of molybdenum, is welded to the electrode I8 adjacent or at the free end thereof and on the surface thereof facing the electrode Il. The envelope I0 has an ionizable gaseous atmosphere therein, such as argon at a pressure of about 25 mm., helium at a pressure of about 100 mm., or a mixture of helium and 0.01 to 5 per cent argon at a pressure of about 100 mm.

The electrode I8 comprises two strips of metal having different coefficients of linear expansion securely fastened together, as by welding. The inner strip has a greater coefficient of expansion than the outer strip and the free end of the electrode I8 moves toward the electrode I1 with increasing temperature and away from the electrode Il with decreasing temperature. While I prefer to make the inner strip of a chrome iron alloy and the outer strip of a nickel iron alloy, it will be understood, of course, that other metals or alloys having the desired characteristics may be used when desired, The electrode I8 and the contact I9 are coated with a material of lower work function, such as zinc, after these elements are welded together and before they are mounted in the envelope I0.

The surface of the stem press around the electrode I1 and the support I6 and between these elements has a coating II thereon of electrically conducting material such as powdered aluminum. Said coating II also covers the lower portion of the electrode I l and the support I8 adjacent the surface of the stem press. Said coating I I is applied to these parts preferably by painting with a mixture of aluminum metal powder, amyl acetate and nitrocellulose lacquer during the manufacture of the switch and before the stem I2 is fused into the envelope I0, as disclosed in my copending application. Said coating Il forms an electrically conducting path having a higher resistance than the conducting path through the ionized gas and the electrodes I I and I8 in the completed switch and its function is to maintain the breakdown voltage of the switch substantially the same in the dark and in the light, as explained in my aforesaid copending application.

As best illustrated in Fig. 2, the U-shaped bimetallic electrode I8 comprises a curved portion and two leg portions, one of which is free to move with respect to the other in response to changes in the temperature of said electrode I8. When the bimetallic electrode I8 fiexes, the leg portions thereof tend to move in opposite directions and the curved portion changes its degree of curvature. As a result, the gap between the curved portion of the electrode I8 and the inner end portion of the electrode I1 does not close when said electrode I8 flexes in response to increasing temperature and, in fact, this gap is called a. fixed gap herein since the change in size thereof, if any, is so small that the effect thereof on the breakdown voltage of the switch is negligible. By making the fixed gap as small or smaller than the gap between the contact I9 and electrode I1 (the contact gap) even when the ambient temperature is as high as approximately C., the breakdown voltage of the switch is determined by the fixed gap at normal temperatures lower than approximately 100 C., and thus the breakdown voltage of the switch is independent of ambient temperature changes encountered in the use of the switch.

The above structure is highly advantageous when the breakdown voltage of the switch changes steeply as the distance between the electrodes changes. Switches filled with argon at a pressure of approximately 25 mm. or helium at a pressure of about 100 mm. have this characteristic and are difficult to manufacture with a desired breakdown voltage when the contact gap rather than a fixed gap determines the breakdown voltage of the switch. My invention permits the use of such gases to obtain switches having a desired breakdown voltage. For example, when the iixed gap of the switch described above is between about 0.12 and 0.5 mm. and the switch is filled with argon or helium at the above pressures, the breakdown voltage is between and volts. Such switches are successfully used in lamp circuits when the voltage of the current source is as low as approximately volts and the lamp operating voltage as high as approximately 120 volts.

The electrode and stem structure illustrated in Fig. 3 of the drawing is similar to that shown in Fig. 2 except for the electrode I'I, the inner end of which is prolonged and bent over the top of the curved portion of the electrode I8. 'I'he other features of the switch incorporating the structure illustrated in Fig. 3 are the same as those described above in connection with Figs. 1 and 2. The smallest gap between the electrodes I'I and I8, which is about 0.12 to 0.5 mm., is between the top of the curved portion of the electrode I8 and the prolongation 2I of the electrode I1.

A further advantage of the switches described above is the enhanced heating of the curved portion of the electrode I8 by the glow discharge covering the closely adjacent electrode I1 which speeds up the closing of the contact gap in the operation of the switch.

The electrodes illustrated in Fig. 4 ci' the drawing are similar to those disclosed in Patent No. 2,280,550, referred to above, and comprises a bimetallic electrode 22 which exes and moves the cam-like part 23 or the contact 2l against the stationary electrode 25 mounted between the legs of the electrode 22 when the latter is heated by a glow discharge between said electrodes. The electrode 22 is welded to current inlead wire 26, the inner portion 2l of which is bent over to extend parallel to the electrode 25. The portion 21 is then part of the electrode22. The gap between the stationary electrode 2'5 and said portion 21 is smaller than the gap between contact 24 and electrode 25 at ambient temperatures as high as about 100 C., and does not change in size in response to changing ambient temperature. Thus the breakdown voltage of a switch incorporating this structure is independent of changes in the ambient temperature encountered 1n use.

The sealed, vitreous envelope and the stem part of the switch illustrated in Fig. 4 have been omitted for simplicity of illustration. As disclosed in Patent No. 2,280,550, when the electrodes 25 and 22 have a material of lower work function, such as magnesium, thereon and the sealed envelope contains a gaseous atmosphere consisting of a mixture of neon and about 0.1 to per cent argon at a pressure of approximately 40 mm., the break.

down voltage of the switch is about 75 volts. This switch is useful in circuits comprising a 110 to 120 volt current source anda lamp having an operating voltage of about 65 volts.

Other means for obtaining a xed gap between the electrodes may be used, when desired. In the structure illustrated in Fig. 4, for example, the straight part of the Contact 24 may be spaced a smaller distance from the stationary electrode 25 than the surface of the cam-like part 23 at ambient temperatures encountered in use, or a plate so spaced may be welded adjacent the center of the stationary electrode 25 and facing the leg portion of the electrode 22 secured to the inlead 26. l

When desired, a radioactive material, such as uranium oxide, compounds of thorium or the like, may be used in the switch envelope, to make the breakdown voltage of the switch substantially the same in the dark and in the light as disclosed in the United States application Serial No. 403,572, filed July 22, 1941, which corresponds to British application No. 13,341,led in England August 22, 1940, inventor Basil N.'Clack, or the envelope may contain, or consist in wholeV or in part, of uranium glass for the same purpose, as disclosed in the copending application of Basil N Clack, Serial No. 385,003, led March 24, 1941, which is assigned to the same assignee as the present application. The coating Il may be omitted or retained, as desired, when such materials or such glass is used. A quantity of finely divided, radioactive uranium oxide on the inner surface of the end of the envelope opposite the stem end thereof is useful for the above purpose.

Other gasesA or gas mixtures at desired pressures may be used in the envelope, other materials of low work function may be used on the electrodes and the size of the fixed gap between the electrodes may be changed for the purpose of obtaining a desired breakdown voltage ofthe switches described above.

From the foregoing, it is'apparent that I have provided a glow-type thermal switch of the type comprising a stationary electrode and a thermally/responsive, bimetallic electrode having an end which is free to move with respect to said stationary electrode in response to changes in ambient temperature over a predetermined temperature range without affecting the breakdown voltage of the switch.

`What I claim as new and desire to secure by Letters Patent of the United States is 1. A glowftype thermal switch designed for operation within a predetermined range of ambient temperature, said switch comprising a sealed envelope containing an ionizable gaseous atmosphere and normally spaced, cooperating electrodes including a stationary electrode and a thermally responsive electrode constructed and arranged to make contact with said'stationary electrode and thus to extinguish the discharge between said electrodes when heated by said discharge, said electrodes having spaced portions providing a xed gap therebetween which is smaller than the gap between the contact portions of said electrodes over said range whereby the breakdown voltage of the switch is independent of changes in ambient temperature within said range.

2. A glow-type thermal switch designed for operation within a predetermined range of ambient temperature, said switch comprising a sealed envelope containing argon at a pressure of approximately 25 mm. and normally spaced, cooperating electrodes including a stationary electrode and a thermally responsive electrode constructed and arranged to make contact with said stationary electrode and thus to extinguish l the discharge between said electrodes when heated by said discharge, said electrodes having spaced portions providing a fixed gap therebetween which is smaller than the gap between the contact portions of said electrodes over said range whereby the breakdown voltage of the switch is independent of changes in ambient temperature within said range.

3. A glow-type thermal switch designed for operation within a predetermined range of ambient temperature, said switch comprising a sealed envelope containing helium at a pressure of about mm. land normally spaced, cooperating electrodes including a stationary electrode and a thermally responsive electrode constructed and arranged to make contact with said stationary electrode and thus to extinguish the discharge between said electrodes when heated by said discharge, said electnodes having spaced portions providing a fixed gap therebetween which is smaller than the gap between the contact portions of said electrodes over said range whereby the breakdown voltage of the switch is independent of changes in ambient temperature Within said range.

4. A glow-type thermal switch designed for operation within a predetermined range of ambient temperature, said switch comprising a sealed envelope containing an ionizable gaseous atmospheranormally spaced, cooperating electrodes and means for maintaining the breakdown voltage of the switch substantially constant in the light and in the dark, said electrodes including a stationary electrode and a thermally responsive electrode constructed and arranged to make contact with said stationary electrode and thus to extinguish the discharge between said electrodes when heated by said discharge, said electrodes having spaced portions providing a fixed gap therebetween which is smaller than the gap between the contact portions oi said electrodes over said range whereby the breakdown voltage of the switch is independent oi changes in ambient temperature within said range.

5. A glow-type thermal switch designed for operation within a predetermined range of ambient temperature, said switch comprising a sealed envelope containing an ionizable gaseous atmosphere, normally spaced, cooperating electrodes and radioactive material, said electrodes including a stationary electrode and a thermally responsive electrode constructed and arranged to make contact with said stationary electrode and thus to extinguish the discharge between said electrodes when heated by said discharge, said electrodes having spaced portions providing a xed gap therebetween which is smaller than the gap between the contact portions of said electrodes over said range whereby the breakdown voltage of the switch is independent oi' changes in ambient temperature within said range.

6. A glow-type thermal switch designed for operation within a predetermined range of ambient temperature, said switch comprising a sealed envelope containing an ionizable gaseous atmosphere, normally spaced, cooperating electrodes, current inlead wires for said electrodes and a body of electrically conducting material providing a connecting path between said wires oi higher resistance than the path through said electrodes and the ionized gas, said electrodes including a stationary electrode and a thermally responsive electrode constructed and arranged to make contact with said stationary electrode and thus to extinguish the discharge between said electrodes when heated by said discharge, said electrodes having spaced portions providing a fixed gap therebetween which is smaller than the gap between the contact portions of said electrodes over said range whereby the breakdown voltage of the switch is independent of changes in ambient temperature within said range.

'7. A glow-type thermal switch designed for operation within a predetermined range of ambient temperature, said switch comprising a sealed envelope containing an ionizable gaseous atmosphere and normally spaced, cooperating electrodes including a stationary electrode and a thermally responsive electrode constructed and arranged to make contact with said stationary electrode and thus to extinguish the discharge bient temperature, said switch comprising ay sealed envelope containing an ionizable gaseous atmosphere and normally spaced, cooperating electrodes including a stationary electrode and a thermally responsive electrode constructed and arranged to make contact with said stationary electrode and thus to extinguish the discharge between said electrodes when heated by said discharge, said electrodes being coated with zinc and having spaced portions providing a xed gap therebetween which is smaller than the gap between the contact portions of said electrodes over said range whereby the breakdown voltage of the switch is independent of changes in ambient temperature within said range.

9. A glow-type thermal switch designed for operation within a predetermined range of ambient temperature, said switch comprising a sealed envelope containing an ionizable gaseous atmosphere and normally spaced, cooperating electrodes including a stationary electrode and a thermally responsive electrode constructed and arranged to make contact with said stationary electrode and thus to extinguish the discharge between said electrodes when heated by said discharge, said electrodes being coated with magnesium and having spaced portions providing a tixed gap therebetween which is smaller than the gap between the contact portions of said electrodes over said range whereby the breakdown voltage o! the switch is independent of changes in ambient temperature within said range.

10. A glow-type thermal switch designed for operation within a predetermined range of ambient temperature, said switch comprising a sealed envelope containing an ionizable gaseous atmosphere and normally spaced, cooperating electrodes including a stationary electrode and a U-shaped, thermally responsive electrode constructed and arranged to make contact with said stationary electrode and thus to extinguish the discharge between said electrodes when heated by said discharge, the curved portion of said U- shaped electrode and the inner end portion of said stationary electrode being spaced and providing a iixed gap between said electrodes, said fixed gap being smaller than the gap between the contact portions of said electrodes over said range whereby the breakdown voltage oi the switch is independent of changes in ambient temperature within said range.

11. A glow-type thermal switch designed for operation within a predetermined range of ambient temperature, said switch comprising a sealed envelope containing an ionizable gaseous atmosphere and normally spaced, cooperating electrodes including a stationary electrode and a U-shaped, thermally responsive electrode constructed and arranged to make contact with said stationary electrode and thus to extinguish the discharge between said electrodes when heated by said discharge, the end portion of said stationary electrode being disposed adjacent the midpart of the curved portion of said U-shaped electrode to provide a xed gap which is the smallest gap between said electrodes over said range whereby the breakdown voltage of the switch is independent of changes in ambient temperature within said range.

12. A glow-type thermal switch designed for operation within a predetermined range of ambient temperature, said switch comprising a sealed envelope containing an ionizable gaseous atmosphere and normally spaced, cooperating electrodes including a stationary electrode and a U-shaped, thermally responsive electrode constructed and arranged to make contact with said stationary electrode and thus to extinguish the discharge between said electrodes when heated by said discharge, said stationary electrode being disposed between the legs of said U-shaped electrode, said U-shaped electrode having a staarranged to make contact with said stationary electrode and thus to extinguish the discharge between said electrodes when heated by said discharge, said electrodes having spaced contact portions providing a variable contact gap and other spaced portions providing a fixed gap spaced from and smaller than said contact gap when the ambient temperature is within said range wherebydthe breakdown voltage of said switch is independent of ambient temperature changes under the condition for which said switch is designed.

LEO R.. PETERS. 

